1. Field of the Invention
The present invention relates in general to an engine for an watercraft. In particular, the present invention relates to a throttle control device for a marine engine.
2. Description of Related Art
An in-line four-cycle engine often powers an outboard motor today in view of emission concerns in the industry. Such an engine includes a plurality of cylinders formed within a cylinder block. The cylinders are arranged above one another along parallel horizontal axes that lie within a common plane. Pistons reciprocate within the cylinders and drive a crankshaft that rotates about a vertical axis.
A cylinder head is attached to an end of the cylinder block. Intake passages are formed within the cylinder head and communicate with the cylinders. In engines employing multiple intake valves, each intake passage diverges into branch passages that communicate with the respective cylinder. These branch passages often are positioned above one another such that the branches diverge to increase a distance between the branch passages in a direction that is generally parallel to the vertical axis about which the crankshaft rotates.
A plurality of butterfly throttle valves typically regulate the amount of air delivered to each cylinder of the engine. The valve disc of each valve is supported within the valve body by a throttle shaft. The throttle valves commonly are arranged on a side of the engine above one another. The throttle valves are all positioned such that such that throttle shafts lie parallel to each other with the throttle valves rotating about an axis that is perpendicular to the rotational axis of the crankshaft.
A throttle linkage commonly interconnects the throttle shafts of the throttle valves. The linkage generally synchronizes the operation of the throttle valves to stabilize engine rotation. The amount of air delivered to the respective cylinder depends upon the opening angle control by the linkage system.
The throttle linkage commonly includes a series of aligned throttle rods which operate a plurality of throttle levers. One end of the throttle lever is connected to the throttle shaft. The levers are placed parallel to one another, and the series of linkage rods join together the outer ends of the levers. Conventional clips connect the linkage rods to the levers.
Prior induction system designs suffer from several shortcomings. For instance, temperature changes often effect the control of the throttle valves. With increased temperature, prior linkage designs thermally expand which causes the positions of the throttle shafts and the corresponding throttle valve to become unsynchronized. For instance, where a linkage rod connects to a relatively fixed first throttle lever at one end and a movable second throttle lever at the other end, thermal expansion of the linkage rod will move the second lever relative to the first lever. The second lever therefore will no longer lie parallel to the first lever, and consequently the corresponding angle of the second throttle valve, which is operated by the second throttle lever, will differ from the angle of the first throttle valve. Thermal expansion of the linkage rod thus unsynchronizes the operation of the first and second throttle valves.
The arrangement of the linkage rods in series compounds this problem. The increased length of each throttle rod stacks up so that the lever operating the last throttle valve typically moves by an amount corresponding to the combined length increase of each throttle rod within the series. The angles of the first and last throttle valves thus can greatly differ so the operation of the charge formers are no longer synchronized and engine revolution becomes unstabilized. When this occurs at low revolution speed (i.e., under idling conditions) the engine can stall.
In addition, when the throttle valves are slightly opened, more air flows over an upper edge of the throttle valve than over a lower edge of the valve. A greater volume of air thus is directed to the upper branch passage than to the lower branch passage. This uneven air flow through the throttle device also causes a richer fuel/air charge to flow into the lower branch passage and lower intake port than through the upper branch passage and upper intake port. Inconsistent combustion consequently results in the respective combustion chamber, degrading engine performance.
The spacing or pitch between the central axes of each cylinder and the spacing or pitch between the center of the throttle valves also are conventionally different within the engine. The induction paths from the throttle valves to the respective cylinders consequently have differing shapes and lengths from one another. The same amount of air therefore is not delivered to each cylinder. This inconsistency produces a power differential between the cylinders, impairing the engine's performance.